1. Field of the Invention
The present invention relates to an identification system, wherein a first unit commonly referred to as a transmitter, interrogator or decoder is operative to identify one or a plurality of second units commonly referred to as responders or transponders.
For the purpose of this specification the first unit will be referred to as an interrogator, such interrogator being operative to generate a signal over a range (for example 1 meter) and to detect a signal generated by a second unit located within the range.
For the purposes of this specification, the second units will be referred to as transponders, each transponder being operative to detect the signal generated by the interrogator, and to emit in response to such detection a signal for reception by the interrogator.
2. Prior Art
Such identification systems are well known and are in general use for access control systems, for animal identification, for vehicle identification and for the identification of valuable goods or partially assembled items on a production line. For example in an access control system, an interrogator is installed near a security door and interrogates transponders in the form of tags carried by personel wishing to gain access to the door.
Application No. UK-A-2112607 discloses an identification system wherein each of a plurality of transponders (referred to in that specification as responders) has a code which uniquely characterizes that transponder, and this code is transmitted by the transponder as a modulation signal of a carrier signal (S.sub.2), in response to reception by the transponder of an interrogation signal (S.sub.1), so that the interrogator can recognise the presence of the transponder. However the system disclosed in UK-A-2112607 cannot deal with the common requirement by users of such systems that more than one transponder shall be recognised simultaneously. This occurs in access control systems when two or more people crowd closely together near a door which is controlled by transponder signals, or when a number of cattle crowd together close to an automatic feeding station or when several goods are loaded together on to a pallet ready for trans-shipment.